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				<h2>8 个有力的 Awk 内建变量</h2>				<h3><em>分类</em> <a href="../w3cnote_genre/code" title="编程技术" >编程技术</a> </h3>
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					<p>Awk 有几个非常强力的内置变量.通常来说，分为两种类型的内置变量: - 第一种是定义的变量可以改变,　比如字段分隔(FS)与记录分隔(RS) - 第二种是可以用来数据处理或者数据总结，比如记录数(NR)与字段数目(NF) 文中 介绍了: <span class="marked">FS,OFS, RS, ORS, NR, NR, FNR</span></p>

<h3>FS: 输入字段分隔符变量</h3>
<p><span class="marked">FS</span>(Field Separator) 读取并解析输入文件中的每一行时，默认按照空格分隔为字段变量,$1,$2...等。<span class="marked">FS</span> 变量被用来设置每一记录的字段分隔符号。<span class="marked">FS</span> 可以是任意的字符串或者正则表达式.你可以使用下面两种方式来声名FS:</p>

<p>使用 <span class="marked">-F</span> 命令选项</p>
<p>作为设置为普通变量使用</p>

<pre>语法:

$ awk -F 'FS' 'commands' inputfilename

或者

$ awk 'BEGIN{FS="FS";}'</pre>
<p><strong>FS</strong> 可以是任意字符或者正则表达式</p>
<p><strong>FS</strong> 可以多次改变, 不过会保持不变直到被明确修改。不过如果想要改变字段分隔符,　最好是在读入文本之前就改变 <span class="marked">FS</span>, 这样改变才会在你读入的文本生效。</p>
<p>下面是一个使用 <span class="marked">FS</span> 读取 /etc/passwd 以 <span class="marked">:</span> 作为分隔符的例子</p>

<pre>$ cat etc_passwd.awk
BEGIN{
FS=":";
print "Name\tUserID\tGroupID\tHomeDirectory";
}
{
&nbsp;&nbsp;&nbsp;&nbsp;print $1"\t"$3"\t"$4"\t"$6;
}
END {
&nbsp;&nbsp;&nbsp;&nbsp;print NR,"Records Processed";
}</pre>
<p>使用结果:</p>

<pre>
$ awk -f etc_passwd.awk /etc/passwd
Name    UserID  GroupID        HomeDirectory
gnats&nbsp;&nbsp;&nbsp;&nbsp;41&nbsp;&nbsp;&nbsp;&nbsp;41&nbsp;&nbsp;&nbsp;&nbsp;/var/lib/gnats
libuuid&nbsp;&nbsp;&nbsp;&nbsp;100&nbsp;&nbsp;&nbsp;&nbsp;101&nbsp;&nbsp;&nbsp;&nbsp;/var/lib/libuuid
syslog&nbsp;&nbsp;&nbsp;&nbsp;101&nbsp;&nbsp;&nbsp;&nbsp;102&nbsp;&nbsp;&nbsp;&nbsp;/home/syslog
hplip&nbsp;&nbsp;&nbsp;&nbsp;103&nbsp;&nbsp;&nbsp;&nbsp;7&nbsp;&nbsp;&nbsp;&nbsp;/var/run/hplip
avahi&nbsp;&nbsp;&nbsp;&nbsp;105&nbsp;&nbsp;&nbsp;&nbsp;111&nbsp;&nbsp;&nbsp;&nbsp;/var/run/avahi-daemon
saned&nbsp;&nbsp;&nbsp;&nbsp;110&nbsp;&nbsp;&nbsp;&nbsp;116&nbsp;&nbsp;&nbsp;&nbsp;/home/saned
pulse&nbsp;&nbsp;&nbsp;&nbsp;111&nbsp;&nbsp;&nbsp;&nbsp;117&nbsp;&nbsp;&nbsp;&nbsp;/var/run/pulse
gdm&nbsp;&nbsp;&nbsp;&nbsp;112&nbsp;&nbsp;&nbsp;&nbsp;119&nbsp;&nbsp;&nbsp;&nbsp;/var/lib/gdm
8 Records Processed</pre>
<h3>OFS: 输出字段分隔符变量</h3>
<p><span class="marked">OFS</span>(Output Field Separator) 相当与输出上的 <span class="marked">FS</span>, 默认是以一个空格字符作为输出分隔符的，下面是一个 <span class="marked">OFS</span> 的例子:</p>


<pre>$ awk -F':' '{print $3,$4;}' /etc/passwd
41 41
100 101
101 102
103 7
105 111
110 116
111 117
112 119</pre>
<p>注意命令中的 print 语句的, 表示的使用一个空格连接两个参数，也就是默认的OFS的值。因此 <span class="marked">OFS</span> 可以像下面那样插入到输出的字段之间:</p>


<pre>$ awk -F':' 'BEGIN{OFS="=";} {print $3,$4;}' /etc/passwd
41=41
100=101
101=102
103=7
105=111
110=116
111=117
112=11</pre>
<h3>RS: 记录分隔符</h3>
<p><span class="marked">RS</span>(Record Separator)定义了一行记录。读取文件时，默认将一行作为一条记录。 下面的例子以 student.txt 作为输入文件，记录之间用两行空行分隔，并且每条记录的每个字段用一个换行符分隔:</p>

<pre>$ cat student.txt
Jones
2143
78
84
77

Gondrol
2321
56
58
45

RinRao
2122
38
37
65

Edwin
2537
78
67
45

Dayan
2415
30
47
20</pre>
<p>然后下面的脚本就会从student.txt输出两项内容:</p>


<pre>$ cat student.awk
BEGIN {
&nbsp;&nbsp;&nbsp;&nbsp;RS="\n\n";
&nbsp;&nbsp;&nbsp;&nbsp;FS="\n";
}
{
&nbsp;&nbsp;&nbsp;&nbsp;print $1,$2;
}
$ awk -f student.awk  student.txt
Jones 2143
Gondrol 2321
RinRao 2122
Edwin 2537
Dayan 2415</pre>
<p>在 student.awk 中，把每个学生的详细信息作为一条记录，　这是因为RS(记录分隔符)是被设置为两个换行符。并且因为 <span class="marked">FS</span> (字段分隔符)是一个换行符，所以一行就是一个字段。</p>
<h3>
ORS: 输出记录分隔符变量</h3>
<p><span class="marked">ORS</span>(Output Record Separator)顾名思义就相当与输出的 <span class="marked">RS</span>。 每条记录在输出时候会用分隔符隔开，看下面的 <span class="marked">ORS</span> 的例子:</p>

<pre>$  awk 'BEGIN{ORS="=";} {print;}' student-marks
Jones 2143 78 84 77=Gondrol 2321 56 58 45=RinRao 2122 38 37 65=Edwin 2537 78 67 45=Dayan 2415 30 47 20=</pre>
<p>上面的脚本，输入文件的每条记录被 <span class="marked">=</span> 分隔开。 附:student-marks 便是上面的输出.</p>
<h3>
NR: 记录数变量</h3>
<p><span class="marked">NR</span>(Number of Record) 表示的是已经处理过的总记录数目，或者说行号(不一定是一个文件，可能是多个)。下面的例子，<span class="marked">NR</span> 表示行号，在 END 部分，<span class="marked">NR</span> 就是文件中的所有记录数目。</p>


<pre>$ awk '{print "Processing Record - ",NR;}END {print NR, "Students Records are processed";}' student-marks
Processing Record -  1
Processing Record -  2
Processing Record -  3
Processing Record -  4
Processing Record -  5
5 Students Records are processed</pre>
<h3>NF:一条记录的记录数目</h3>
<p><span class="marked">NF</span>(Number for Field)表示的是，一条记录的字段的数目.　它在判断某条记录是否所有字段都存在时非常有用。 让我们观察 student-mark 文件如下:</p>

<pre>$ cat student-marks
Jones 2143 78 84 77
Gondrol 2321 56 58 45
RinRao 2122 38 37
Edwin 2537 78 67 45
Dayan 2415 30 47</pre>
<p>接着下面的Awk程序，打印了记录数(<span class="marked">NR</span>),以及该记录的字段数目: 因此可以非常容易的发现那些数据丢失了。</p>


<pre>$ awk '{print NR,"-&gt;",NF}' student-marks
1 -&gt; 5
2 -&gt; 5
3 -&gt; 4
4 -&gt; 5
5 -&gt; 4</pre>
<h3>FILENAME: 当前输入文件的名字</h3>
<p><span class="marked">FILENAME</span> 表示当前正在输入的文件的名字。 AWK 可以接受读取很多个文件去处理。看下面的例子:</p>


<pre>$ awk '{print FILENAME}' student-marks
student-marks
student-marks
student-marks
student-marks
student-marks</pre>
<p>在输入的文件的每一条记录都会输出该名字。</p>

<h3>FNR: 当前输入文件的记录数目</h3>
<p>当awk读取多个文件时，<span class="marked">NR</span> 代表的是当前输入所有文件的全部记录数，而 <span class="marked">FNR</span> 则是当前文件的记录数。如下面的例子:</p>


<pre>$ awk '{print FILENAME, "FNR= ", FNR,"  NR= ", NR}' student-marks bookdetails
student-marks FNR=  1   NR=  1
student-marks FNR=  2   NR=  2
student-marks FNR=  3   NR=  3
student-marks FNR=  4   NR=  4
student-marks FNR=  5   NR=  5
bookdetails FNR=  1   NR=  6
bookdetails FNR=  2   NR=  7
bookdetails FNR=  3   NR=  8
bookdetails FNR=  4   NR=  9
bookdetails FNR=  5   NR=  10</pre>
<p>附: bookdetails 与 student-marks 内容一样，作例子. 可以看出来 <span class="marked">NR</span> 与 <span class="marked">FNR</span> 的区别。</p>

<p>经常使用 <span class="marked">NR</span> 与 <span class="marked">FNR</span> 结合来处理两个文件，比如有两个文件:</p>


<pre>$ cat a.txt
李四|000002
张三|000001
王五|000003
赵六|000004

$ cat b.txt
000001|10
000001|20
000002|30
000002|15
000002|45
000003|40
000003|25
000004|60</pre>
<p>如果想作对应的话,　比如张三|000001|10</p>

<pre>$ awk -F '|' 'NR == FNR{a[$2]=$1;} NR&gt;FNR {print a[$1],"|", $0}' a.txt b.txt
张三 | 000001|10
张三 | 000001|20
李四 | 000002|30
李四 | 000002|15
李四 | 000002|45
王五 | 000003|40
王五 | 000003|25
赵六 | 000004|60</pre>


<blockquote><p>英文原文：http://www.thegeekstuff.com/2010/01/8-powerful-awk-built-in-variables-fs-ofs-rs-ors-nr-nf-filename-fnr/</p>
<p>译文 ：http://shomy.top/2016/05/05/trans-8-powerful-awk-built-in-variables/</p></blockquote>

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